Lake

Aside from the natural aging process, major threats to the longevity of lake fertility include pollution (including acid rain), eutrophication, and shoreline overdevelopment. Acid rain is formed by sulfates and nitrates emitted from coal-burning industries and automobile exhaust pipes. These chemicals combine with moisture and sunlight and are converted into sulfuric and nitric acid that enter lakes via precipitation. Acid rain has a pH of four and half, contrasting with the normal rain pH value of 5.6. Since a single digit pH difference (say, from eight to nine) represents a 10-fold change in acidity, acid rain is more than 10 times more acidic than normal rain. Freshwater life generally prefers alkaline (basic, non-acidic) conditions, but lake fertility is usually fairly functional down to a pH of six. However, when pH drops to five and below, as the effects of acidrain accumulate, life forms are severely effected. Plants, plankton, insects, and fish all gradually disappear. Young and old organisms die first, followed by the young and middle-aged adults. Many bacteria even die. Other chemical pollutants include fertilizers and pesticides that drain into lakes through soil and enter through streams. Pesticides are toxic to fish, while fertilizers can cause eutrophication.

Eutrophication is the abundance of nutrients for fertile growth. It is a natural phenomenon in mature lakes. However, chemical pollutants, including phosphorous and nitrogen compounds, can artificially propel lakes to this state where the demand by aquatic animals on lake oxygen is great. Man-made eutrophication threatens to deplete lake oxygen which can kill most of a lake's fish. Some eutrophigenic lakes are now aerated by man to increase available oxygen.

Shore overdevelopment disrupts natural habitats and increases pollution. Shorelines that are built up with dirt to support construction of buildings can crush wet, rocky areas that some lake species use for spawning. In addition, shoreline plant life is sometimes removed to create sandy, recreational areas, and the influx of people usually increases pollution.